Dr Yeomans said: "Since regular sky surveys began in the 1990s, we've never seen an object this big get so close to Earth."

The rock in question is believed to be made of stone, rather than metal or ice and Yeomans estimates an asteroid of that sort flies past Earth on average every 40 years, but is only likely to strike the planet ever 1,200 years or so.

An estimated 500,000 near-Earth objects measuring up to 98ft (30m) are believed to be undiscovered.

Dr Detlef Koschny, also from the SSA, said: "We are developing a system of automated optical telescopes that can detect asteroids just like this one, with the goal of being able to spot them at least three weeks before closest approach to Earth."

Phew.

Astounding asteroid craters

Astounding asteroid craters

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Artist's impression of giant planetoid hitting Earth

Artist's impression of a planetoid 1000 km wide (about the distance from New York to Chicago) hitting a young Earth. Donald Davis/NASA